Motors for motor-driven vehicles such as hybrid vehicles and electronic vehicles are driven by alternating current power generated by an inverter. An inverter generally includes power transistors such as IGBTs (Insulated Gate Bipolar Transistors) configured in three-arm bridge connection, and these power transistors are gate-controlled to perform switching, thereby converting direct current power to alternating current power.
Patent Document 1 discloses a structure in which the temperature of an IGBT is detected and, using the output command current, the battery voltage, and the IGBT temperature as parameters, a resistance value of the gate resistor is controlled such that a voltage including a surge voltage to be applied to the IGBT attains a predetermined value which is below an allowable breakdown voltage.
Further, Patent Document 2 discloses adjusting the gate voltage in accordance with the IGBT temperature and the direct current side voltage information, and more specifically discloses that the gate voltage is set low in order to reduce noise when the temperature of coolant of an element is low and a thermal margin exists and that the gate voltage is set high in order to reduce the switching loss when the direct current side voltage of the inverter is high.
FIG. 14 illustrates a structure of conventional technology. FIG. 14 is a view illustrating a structure of an inverter device for driving a motor. A direct current voltage of a battery 1 is converted by an inverter device 2 into a three-phase alternating current voltage having variable voltages and variable frequencies, which is then supplied to an AC motor 3. The inverter device 2 includes an inverter main circuit 21, a voltage smoothing capacitor 22, and an inverter controller 23. The inverter main circuit 21 is formed of IGBTs 211 in a three-phase bridge connection configuration, and each IGBT includes an anti-parallel diode 212. The inverter controller 23 includes a gate-driving circuit 231 which applies a gate voltage to the IGBTs 211 to drive the IGBTs 211 ON and OFF. Further, a voltage-adjustable gate power source circuit 232 is provided for adjusting the output voltage of the gate-driving circuit 231, and the output voltage thereof is controlled by a gate power source voltage control circuit 233. A voltage command is applied to the gate power source voltage control circuit 233 from a voltage command device 234. The voltage command device 234 acquires an output of a voltage detector 235 which detects a direct current voltage of the inverter main circuit 21 and an output of a temperature detector 238 which detects a temperature of a coolant which cools the IGBT 211 or those IGBTs, and determines the voltage command. The voltage detector 235 detects the voltage between direct current side terminals P and N of the inverter main circuit 21; i.e., the voltage of the voltage smoothing capacitor 22. When the direct current voltage is high, which means that the switching loss is also great, the gate voltage is increased to reduce the loss. Further, when the output of the temperature detector 238 which detects the temperature of the IGBT 211 and the coolant is low and a margin exists in the thermal loss of the IGBT 211, the gate voltage is increased to reduce the switching noise of the element.
Further, Patent Document 3 discloses detecting the temperature of a power semiconductor switching element and varying the gate voltage or the gate resistance based on the detected value, to thereby reduce the temperature dependency of the element loss. Specifically, Patent Document 3 discloses increasing the gate voltage or reducing the gate resistance when the element temperature is high.    Patent Literature 1: JP 2001-169407 A    Patent Literature 2: JP 2007-89325 A    Patent Literature 3: JP 2007-259576 A